nebraska: A lightweight mock of the Omaha server

Simple server that responds to POST requests with an error message and
echos the request body.

BUG=chromium:892450
TEST=Manual, unittesting

Change-Id: I1ab5f27eeae32f0da92f3d781897e4f5b1d4d89f
Reviewed-on: https://chromium-review.googlesource.com/1279130
Commit-Ready: ChromeOS CL Exonerator Bot <chromiumos-cl-exonerator@appspot.gserviceaccount.com>
Tested-by: Colin Howes <chowes@google.com>
Reviewed-by: Amin Hassani <ahassani@chromium.org>
diff --git a/run_unittests b/run_unittests
index c95ea93..22019fc 100755
--- a/run_unittests
+++ b/run_unittests
@@ -42,7 +42,7 @@
 fi
 
 # Invoke unit test scripts.
-for unittest_script in *_unittest.py; do
+for unittest_script in *_unittest.py nebraska/*_unittest.py; do
   echo "Running $unittest_script":
   ./${unittest_script} || test ${force_all} || break
 done